The present invention relates to video displays and more particularly, to a method and system for improving the image quality of a display in which a pixel is illuminated by pulses generated in subfields of a frame of the image in accordance with a pulse distribution function. A maximum pixel value to be imaged during the frame is determined, and the pulse distribution is modified based on the maximum pixel value. The invention is particularly suited for use with plasma display panels.
Digital displays such as alternating current (AC) Plasma Display Panels (PDPs) are evolving as an attractive choice to view television programming, especially with regard to the emerging digital television and high definition television (DTV/HDTV) formats. Conventional cathode ray tubes (CRTs) have an established high picture quality, and PDPs are striving to achieve a similar quality in order to attract widespread consumer acceptance.
PDPs, i.e., gas discharge panels, are well known in the art and, in general, comprise a structure including a pair of substrates respectively supporting column and row electrodes, each coated with a dielectric layer and disposed in parallel spaced relation to define a gap therebetween in which an ionizable gas is sealed. The substrates are arranged such that the electrodes are disposed in orthogonal relation to each other, thereby defining points of intersection which, in turn, define discharge pixel sites at which selective discharges may be established to provide a desired storage or display function.
It is known to operate such panels with AC voltages and particularly to provide a write voltage which exceeds a firing voltage at a given discharge site, as defined by selected column and row electrodes, thereby to produce a discharge at a selected cell. The discharge can be continuously xe2x80x9csustainedxe2x80x9d by applying an alternating sustain voltage, which, by itself, is insufficient to initiate a discharge. The technique relies upon wall charges generated on the dielectric layers of the substrates which, in conjunction with the sustain voltage, operate to maintain continuing discharges.
Referring to FIG. 1, the structure of a full color AC plasma panel is schematically illustrated. Plasma panel 410 includes a back substrate 412 upon which plural column address electrodes 414 are supported. Column address electrodes 414 are separated by barrier ribs 416 and are covered by red, green and blue phosphors 418, 420 and 422, respectively. A front transparent substrate 424 includes a-pair of sustain electrodes 426 and 428 for each row of pixel sites. A dielectric layer 430 is emplaced on front substrate 424 and a magnesium oxide overcoat layer 432 covers the entire lower surface thereof, including all of sustain electrodes 426 and 428.
The structure of FIG. 1 is sometimes called a single substrate AC plasma display since both sustain electrodes 426 and 428, for each row, are on a single substrate of the panel. An inert gas mixture is positioned between substrates 412 and 424 and is excited to a discharge state by sustain voltages applied by sustain electrodes 426 and 428. The discharging inert gas produces ultra-violet light that excites the red, green and blue phosphor layers 418, 420 and 422, respectively to emit visible light. If the driving voltages applied to column address electrodes 414 and sustain electrodes 426, 428 are appropriately controlled, a full color image is visible through front substrate 424.
In order to cause the AC plasma panel of FIG. 1 to exhibit a full color image for applications such as television or computer display terminals, a means of achieving a gray scale is needed. Since it is desirable to operate AC plasma panels in a memory mode to achieve high luminance and low flicker, an addressing technique is utilized to achieve image gray levels in pixels that only exist in the ON or OFF states. Such addressing technique is described by Yoshikawa et al. in xe2x80x9cA Full Color AC Plasma Display With 256 Gray Scalexe2x80x9d, Japan Display, 1992, pp. 605-608. Because a PDP is a digital device, it can provide only affixed number of gray scale gradations. In the case of an 8-bit red-green-blue (RGB) signal, 256 gradations are possible.
FIG. 2 illustrates the driving sequence used by Yoshikawa et al. to achieve a 256 gray scale. The drive sequence is sometimes called the sub-field addressing method. The plasma. display panel is addressed in a conventional video manner that divides images into frames. A typical video image may be presented at 60 frames per second, which corresponds to a frame time of 16.6 milliseconds. The sub-field addressing method shown in FIG. 2. divides each frame into 8 sub-fields, SF1-SF8.
As shown in FIG. 3,.each of the 8 sub-fields is further divided into an address period and a sustain period. During the sustain period, a sustain voltage is applied to sustain electrodes 426 and 428, shown in FIG. 1. Thus, if a given pixel site is in the ON state, it is caused to emit light by one or more sustain pulses. By contrast, the sustain voltage is insufficient to cause a discharge at any pixel site that is in the OFF state.
Note in FIG. 2 that the length of the sustain period of each of the 8 sub-fields is different. The first sub-field has a sustain period with only 1 complete sustain cycle period. The second sub-field has 2 sustain cycles, the third sub-field has a sustain period with 4 sustain cycles and, so forth, until the 8th sub-field which has a sustain period with 128 sustain cycles.
By controlling the sustaining of a given pixel site that has been addressed, the perceived intensity of the pixel site can be varied to any one of the 256 gray scale levels. Suppose it is desired for a selected pixel site to emit at half-intensity or at level 128 out of 256. In such a case, referring to FIG. 1, a selective write address pulse is applied to the pixel site during sub-field 8 by applying an appropriate voltage to a column address electrode 414, and utilizing one of sustain lines 426, 428 as the opposing address conductor. No address pulses are applied during the other sub-fields to the addressed pixel site. This means that during the first 7 sub-fields, there is no writing action and therefore no light is emitted during the sustain periods. However, for sub-field 8, the selective write action turns ON the selected pixel site and causes an emission of light therefrom during the sub-field 8 sustain period, in this case for 128 sustain cycles. The 128 sustain cycle per frame energization corresponds to a half-intensity for a frame time.
If, alternatively, it is desired for the selected pixel site to emit at one-quarter intensity or at level 64 out of 256, then a selective write address pulse is applied to the pixel site during sub-field 7 and no address pulses are applied during the other sub-fields. Thus, during sub-fields 1, 2, 3, 4, 5, 6 and 8, there is no writing and therefore no light is emitted during the respective sustain periods. However, for sub-field 7, the selective write turns ON the selected pixel site and causes an emission of light during the sub-field sustain period (in this case, for 64 sustain cycles corresponding to a 1-quarter intensity). For a full-intensity case, the selective write address pulse is applied during all 8 sub-fields so that the pixel site emits light for all sustain periods for each of the 8 sub-fields, corresponding to a full-intensity for the frame.
The Yoshikawa et al. procedure enables any of 256 different intensities to be achieved-through the action of a display processor supplying an 8-bit data word for each sub-pixel site, the data word corresponding to the desired gray intensity level. By routing each of the bits of the data word to control the selective write pulse of each of the 8 address periods of the 8 sub-fields in a given frame, the 8-bit data word controls the number-of sustain cycles during which the selected pixel site will emit light for that frame. Thus, any integer number of sustain cycles per frame between and including 0-255 is obtainable.
FIG. 4 shows a standard sustain pulse distribution over 8 subfields for an 8-bit grayscale. In an 8-subfield system, the sustain pulse distribution is binary-weighted. That is, each subsequent subfield will contain twice the number of pulses as the previous subfield.
However, a PDP system is not limited to 8 subfields per frame. Japanese Patent No. Application No. 10-176863 describes a system in which the pulses for the 8-bit grayscale are distributed over 12 subfields. FIG. 5 shows an example of a 12-subfield sustain pulse distribution for an 8-bit grayscale, similar to that described in the ""863 application.
Japanese Patent Application No. 08-311647 describes another distribution known as pulse width modulation (PWM) coding. FIG. 6 shows an example of a PWM 12-subfield sustain pulse distribution for an 8-bit grayscale. The example in FIG. 6 is similar to that in FIG. 5, but FIG. 6 relates to PWM and assigns different weights to the subfields.
Conventional video signals are gamma corrected to rectify non-linearities of color cathode ray tubes. However, PDPs do not exhibit such nonlinearities. Accordingly, in order to use a conventional video signal in a PDP system, an xe2x80x9cinversexe2x80x9d gamma function must remove the gamma correction curve embedded in the conventional video signal and produce an output that matches the linearity of the PDP. The linear output data is represented in an 8-bit field that is sent to display logic circuitry for subfield processing.
The inverse gamma function applied to the gamma corrected input data is typically defined by the equation:                                           Output            xe2x80x94                    ⁢          Data                =                              Input            xe2x80x94                    ⁢          Range          xc3x97                                    (                                                                    Input                    xe2x80x94                                    ⁢                  Code                                                                      Input                    xe2x80x94                                    ⁢                  Range                                            )                        2.2                                              (        1        )            
FIG. 7 is a graph representing the gamma correction function (Curve B), the inverse gamma function (Curve C) and a desired linear output function (Curve A). Inverse-gamma correction greatly reduces the number of gradations represented on the display. While the linear response allows 256 different output values, the inverse-gamma curve allows only 184 different output values. This is most evident in the low-level image data where the input value must change considerably to achieve a small change in the output value. As the input value increases, the slope of the curve increases, so that at high input levels a small change of input produces a large change of brightness.
FIG. 8 is a graph of the gamma correction function for input values ranging from 0 to 40 counts of conventional video signal data. Note that, an input value of 15 is required before any change is produced at the output, and input values of 16 through 25 all produce-an output value of 1. Consequently, at low intensity levels, a viewer sees a set of wide contours, each consisting of a single value decoded from a larger number of input values.
A display controller for a PDP receives the gamma corrected input data, applies the inverse gamma function and enables individual subfields to produce a desired level of luminance. Since different types of digital displays produce different amounts of light and may have different brightness requirements, the amount of light produced varies. This requires use of a scaling operation to weight the subfields to yield full intensity. To preserve the linearity of the display, the subfields are binary coded, i.e., each subfield produces twice the light as the previous subfield, as described above. When the number of pulses in each subfield is scaled to meet a brightness requirement, the binary weighting is scaled. For example, to increase the brightness by 5 times, quantities of 5, 10, 20, 40, 80, 160, 320, and 640 sustain pulses are implemented in subfields 1 through 8, respectively.
These prior art techniques for managing the intensity of an image on a PDP suffer from several limitations. First, as low light level information is intensified, intensity contouring is visible when an image presents data that moves between low level intensities. Second, the gradual slope of the inverse-gamma function for low input values produces artifacts that are perceptible to the human eye. The human eye operates more logarithmically than linearly and consequently, it readily perceives a change in low light levels, making a viewer highly receptive to low level intensity transitions. Third, a moving picture disturbance (MPD) occurs as light shifts between subfields in a moving image. This causes the viewer to see false color contours as an image shifts across a display.
As discussed above, a pixel that is to be illuminated in a subfield is first activated by a write voltage applied to the electrodes that define the pixel. Nonetheless, the pixel is addressed and sustain pulses are generated regardless of whether the pixel is to be illuminated. The addressing of the pixel and the generation of sustain pulses in a subfield within which a pixel will not be illuminated is a waste of power.
It is an object of the present invention to provide a method and system for improving the image quality of a display in which a pixel is illuminated by pulses generated in subfields of a frame of the image in accordance with a pulse distribution function.
It is another object of the present invention to provide such a method and system that improves resolution at low intensity levels.
It is another object of the present invention to provide such a method and system that reduces moving picture disturbances.
It is yet another object of the present invention to provide such a method and system that reduces power applied to the display.
In accordance with a first method of the present invention, a method is provided for improving an image on a display that images pixels. Each of the pixels has an intensity represented by a respective pixel value, an intensity of a given pixel being associated with a number of pulses produced within a set of subfields in a frame-time, and the pulses allocated among the set of subfields in accordance with a pulse distribution. The method comprises the steps of determining a maximum pixel value to be imaged during the frame-time, and altering a number of pulses within a given subfield based on the maximum pixel value, thus modifying the pulse distribution.
In accordance with a second method of the present invention, a method is provided for reducing power consumed by a display that images pixels in which an intensity of a given pixel is associated with a number of pulses produced within a set of subfields in a frame-time. The method comprises the step of reducing power to the display during a given subfield in which none of the pulses are applied to produce the intensity of the given pixel.
The invention takes advantage of subfields that would not ordinarily be used to produce the desired level of luminance. The maximum pixel value is compared to a threshold that correlates to a sustain pulse distribution boundary of a subfield. The threshold is related to a number of pulses allocated to subfields prior in time in a frame-time. In the preferred embodiment, the invention identifies the subfield having the smallest associated threshold that is also greater than the maximum pixel value. When the maximum pixel value is less than a threshold, subfields occurring after that threshold can be used for the production of new pulses or for a redistribution of existing pulses. Also, an unused subfield can provide a period of time during which power to the display can be reduced.